A variety of cam locking or “self-tightening” fasteners have been disclosed in the prior art. Such fasteners accommodate a workpiece exposed to vibration. Some prior art devices may be found in U.S. Pat. No. 3,263,727 which issued Aug. 2, 1966 to Arthur B. Herpolsheimer; U.S. Pat. No. 3,417,802 which issued Dec. 24, 1968 to Carl O. Oldenkott; and Canadian Pat. No. 1,320364 which issued Jul. 20, 1993 and to the present inventor and is marketed commercially under the trademark DISC-LOCK.
Such devices usually have one or more washer-shaped pieces having inclined cams on one side and a series of ridges on the other. The washers are mounted so that the cam surfaces will mate. In a typical situation, the washers are mounted on a stud between a nut and the workpiece.
Vibration or shock will cause the stud or bolt to elongate. The nut tends to rotate loose. A cam locking device prevents this since the cam rise angle is greater than the lead angle of the thread on the bolt. As the nut rotates relative to the washer, the preload is actually increased, further locking the nut.
One shortcoming of the prior art and commercially available self-tightening fasteners is that the stacked nut and washer have a combined height which presents a relatively large profile preventing these fasteners from being used in certain situations, e.g. where there is a small fastener clearance dimension.
In my prior invention, the self-tightening fastener's washer is captured by an integral skirt that extends from the bottom of the nut. To date, this skirt has always had a thin wall construction to facilitate a swaging operation which flares over the lower end to retain the washer. This thin wall construction, however, is not conducive to threading. Therefore, to provide an adequate number of threads within the fastener, my prior invention had a fastener height to washer diameter ratio of at least 0.54 and more typically about 0.65 (e.g., a height to washer diameter ratio of approximately 0.70 for a ½″ fastener). While my prior fastener was adequate for some applications, the relatively high profile prevented its use in other applications.
There was therefore a need for a cam locking fastener having both a captive washer and a low profile.